1. Field of the Invention
This invention relates to a biocompatible ventricular assist and arrhythmia control device, and more particularly to such a device which (1) can be completely and readily implanted external to the patient's heart, thereby avoiding thrombogenesis and other complications which may arise from contact between the blood flow and artificial, nonbiological surfaces; (2) is of relatively simple, light-weight, and compact construction; (3) requires a relatively small amount of energy to provide reliable, long-term support of native mechanical cardiac function; (4) contains control means to determine if left ventricular stroke volume and/or pressure is adequate and to change the compressive force as needed, thereby assuring an adequate flow of oxygenated blood; (5) includes bradycardic and tachyarrhythmic (pacing and cardioverting/defibrillating) control features, which will facilitate device operation in synchronism with left ventricular contraction; and (6) further enhances the patient's quality of life by providing transcutaneous recharging of the implanted power source and by maximizing the time spent free of the device's external battery pack.
2. Description of the Prior Art
The need for an improved ventricular assist device has long been apparent. The pool of patients suffering from congestive heart failure (CHF), a progressive disease often precipitated by acute myocardial infarction, continues to grow. In 1983 alone the estimated incidence of CHF, in the course of which the heart's mechanical pumping action is severely compromised, was 400,000 in the United States. Some 2.3 million or more persons suffer from varying degrees of the disease, with the estimated annual death rate from mechanical cardiac dysfunction being 165,000. Individuals with worsening CHF who otherwise would be expected to have years of productive life ahead of them, are generally regarded as candidates for a ventricular assist system. At present, however, no generally recognized safe and effective assist device is available.
Another patient group potentially in need of mechanical heart assistance consists of cardiac surgery patients who otherwise would die from profound refractory heart failure after removal of cardiopulmonary bypass. The intra-aortic balloon has been used to assist the circulation mechanically when other therapies have failed to allow weaning from cardiopulmonary bypass. However, half of these assisted patients die from cardiogenic shock (heart failure) despite the intra-aortic balloon. Therefore, a need exists for a more effective form of mechanical circulatory support that is capable of maintaining the systemic circulation and unloading the left ventricle while native myocardial function recovers.
A third patient group requiring mechanical heart assistance are those tachyarrhythmia patients who are at risk of sudden death due to electrical cardiac dysfunction but who also are at risk from mechanical heart failure.
In sum, it is estimated that a safe and effective implantable heart assist device could save the lives of 100,000 or more patients a year; some estimates go as high as 230,000.
In most cases, the underlying causes of the heart's weakened condition are coronary artery disease and its sequelae. The majority of these patients have a normal right ventricle but a left ventricle that has been damaged in specific regions by partial or complete arterial blockages. Ideally, then, a device designed to assist the failing heart should be able to supplement the heart's workload and also compensate for or support weakened portions of the left ventricular wall, including the apex or interventricular septum. Such a device might also incorporate pacemaker and implantable cardioverter/defibrillator technology for treating those patients who also suffer from such electrical dysfunctions as bradycardia or tachyarrhythmias.
Present ventricular assist devices (VADs) and artificial hearts have not met these needs. Existing devices generally feature blood flow pathways made from nonbiological materials. These materials (e.g., acrylics, TEFLON (polytetrafluorethylene), silicone rubber) often damage blood cells and blood proteins and produce clots, thereby presenting a generic risk of downstream lodgment (thromboembolism) in the circulatory system; attempts to coat plastics with heparin, an anticoagulant, have not been successful on a long-term basis. In fact, blood clots cause most of the deaths reported after implantation of an artificial heart or assist device. Depending on where lodgment occurs, blood clots may cause strokes, kidney failure, death (necrosis) of the intestinal wall or peritonitis, or equally severe damage to other organs.
In addition, cardiac arrhythmias may develop during ventricular assistance and adversely affect blood flow. Present assist devices do not treat these electrical dysfunctions. Other problems with existing assist devices include their substantial weight and the fact that they displace a large volume in the patient's body, which complicates implantation and increases the risk of other complications.
Another significant difficulty involves energy supply and consumption. Because current VADs lack a satisfactory implantable energy source, they must be continuously powered percutaneously. This produces a high risk of infection and generates psychological problems for the patient, who must be constantly tethered to an external power source. Some VADs now under development may offer rechargeable implanted batteries coupled with continuous electromagnetic energy transmission through the skin, the external energy source being a series of nickel cadmium batteries placed in a vest or belt. Other state-of-the-research-art VADs may allow the patient to remove the vest or belt for a brief period--up to 20-30 minutes, for example. However, greater freedom from external device dependence continues to be constrained because the implantable batteries used in these devices have a limited number of recharge cycles, poor state-of-charge indicators, poor energy density, and poor energy retention, particularly at body temperature.
Previous attempts to provide ventricular assistance have ranged from artificial hearts (e.g., the Jarvik-7), to devices which directly pump the blood via an artificial pathway inserted through the ventricular wall, to devices which exert pressure on the outside of the heart. Most frequently, pressure-exerting devices involve some form of flexible bladder within a support structure such that expansion of the bladder presses on the ventricle and facilitates expulsion of blood. See, for example, U.S. Pat. Nos. 3,587,567 to Schiff; 3,371,662 to Heid et al.; 4,048,990 to Goetz; and 4,192,293 to Asrican. Another structurally related device (U.S. Pat. No. 4,506,658 to Casile) envisions a truncated conical structure of sac-lined rigid panels separated by contractible and expandable sections.
In all of these proposed devices, the support structure encases all or most of the heart and either pushes against or otherwise contacts the right as well as the left ventricle. This complicates ventricular assistance since most cases of heart failure are due to a failure of the left ventricle, not the right. The right ventricle, which pumps against a pressure that is typically one-fifth of that seen by the left, is generally capable of proper function without assistance. Accordingly, these devices risk preferentially pumping blood from the right ventricle, as a consequence of which blood would accumulate in the lungs and cause pulmonary edema. In recognition of this difficulty, one recent proposal (U.S. Pat. No. 4,536,893 to Parravicini) envisions using two segmented sacs, selectively fed by a pumping fluid, to compress the ventricles separately.
Bladder systems have additional shortcomings. These include the possibility of catastrophic bladder fluid leakage, a propensity for damaging the heart surface due to poor fixation and/or rubbing of the bladder against the heart's surface, and the unnatural convex form presented to the heart's surface during systolic bladder expansion.
Another type of cardiac assist system is designed to compress all or part of the heart by alternately tightening and releasing a circumferential compression band. For example, one proposed system for body organs (U.S. Pat. No. 4,304,225 to Freeman) involves a flexible strap which is fixed to a contoured plastic block and which would pass across the back of the heart. In response to electrical pulses, a motor assembly would alternately reel in and release the flexible strap, thereby forcing fluid from the subject organ. One liability of this approach is that a pressure of between 20 and 70 mm Hg in the volume under the strap would pump blood from the right ventricle but not the left, since 70 mm Hg or more is required for blood to exit the left ventricle into the aorta. As with the bladder-type devices discussed above, such a preference could lead to a buildup of blood in the lungs, producing severe pulmonary complications.
U.S. Pat. No. 4,583,523 to Kleinke and Freeman illustrates a heart assist mechanism with some similarities to the present invention. However, there are numerous differences. For example, Pat. No. 4,583,523 compresses the aorta, not the left ventricle, and it compresses during the diastolic phase of cardiac contraction instead of the systolic phase. Furthermore, it has no means to continuously control the depth of stroke. Specifically, there is no means to monitor the adequacy of left ventricular stroke volume.